In Situ Infrared Study of 4,4'-Bipyridine Adsorption on Thin Gold Films

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In Situ Infrared Study of 4,4′-Bipyridine Adsorption on Thin Gold Films Th. Wandlowski,* K. Ataka, and D. Mayer Institute for Thin Films and Interfaces ISG3, Research Center Ju¨ lich, 52425 Ju¨ lich, Germany Received January 28, 2002. In Final Form: March 11, 2002 The phase formation of 4,4′-bipyridine (4,4′-BP) and its coadsorption with interfacial water on quasiAu(111) film electrodes (20 nm) from 0.05 M KClO4 has been studied employing in-situ surface enhanced infrared reflection adsorption spectroscopy (SEIRAS). Organic molecules form, dependent on the electrode potential and in the absence of Faradaic reactions, three monolayers of distincly different orientation. The high coverage adlayer I is composed of perpendicularly oriented 4,4′-BP molecules coordinated with one nitrogen atom to the underlying positively charged electrodes (C2v symmetry). Changing the electrode potential toward negative values causes two first-order phase transitions giving rise to two low-coverage organic adlayers. These transitions are accompanied by an in-plane tilting of the N-coordinated molecule. The conclusions on the interfacial orientation of 4,4′-BP are supported by a comparative analysis of in situ SEIRAS, surface enhanced Raman spectroscopy, and sum frequency generation spectra. SEIRAS spectra also indicate that 4,4′-BP is coadsorbed with water molecules and modifies the interfacial hydrogenbonded network of the later. The nature of these interactions and their consequences for the creation of functionalized adlayers on metal electrodes are compared with related N-heterocyclic molecules.

1. Introduction The electrochemical adsorption of organic molecules is characterized by (1) interactions with the electrode through image charges and/or substrate-adsorbate coordination, (2) displacement of previously adsorbed solvent molecules and/or electrolyte ions, (3) hydrophobic and hydrophilic interactions with remaining solvent molecules, and (4) interactions between the adsorbate species. The latter may involve dispersion and dipole-dipole coupling, π-stacking, and/or hydrogen bonding.1 Vibrational spectroscopy, such as infrared reflection adsorption spectroscopy (IRAS),2 sum frequency generation (SFG) spectroscopy,3 surface-enhanced Raman spectroscopy (SERS),4 and/or surface-enhanced infrared adsorption spectroscopy (SEIRAS)5 are powerful techniques for in situ investigations on structure and reactivity aspects of complex interfacial processes under steady-state as well as under dynamic conditions. The combination of SEIRAS and/or SERS with an attenuated total reflection (ATR) configuration provided new insight on the identity and reactivity of interfacial species, such as water, organic molecules, or ions, under potential controlled conditions at defined interfaces.6-10 In 1980 Hartstein et al. reported for the first time that the IR absorption of organic molecules, deposited on thin * To whom correspondence may be addressed: e-mail, [email protected]; fax, 49 2461 61 3462; telephone, 49 2461 61 3907. (1) Guidelli, R. In Adsorption of Organic Molecules; Lipkowski, J., Ross, P. N., Eds.; VCH: New York, 1992; p 1. (2) Iwasita, T.; Nart, F. C. Prog. Surf. Sci. 1997, 55, 271. (3) Tadjeddine, A.; Le Rille, A.; Pluchery, O.; Vidal, F.; Zheng, W. Q.; Peremans, A. Phys. Status Solidi A 1999, 175, 89. (4) Tian, Z. Q.; Ren, B. In Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; J. Wiley & Sons: New York, 2001; p 9162. (5) Osawa, M. Bull. Chem. Soc. Jpn. 1997, 70, 2861. (6) Ataka, K.; Osawa, M. Langmuir 1998, 14, 951. (7) Watanabe, M.; Zhu, Y.; Uchida, H. J. Phys. Chem. B 2000, 104, 1762. (8) Noda, H.; Wan, L. J.; Osawa, M. Phys. Chem. Chem. Phys. 2001, 3, 3336. (9) Bruckenbauer, A.; Otto, A. J. Raman Spectrosc. 1998, 29, 665. (10) Futamata, M.; Bruckenbauer, A. Chem. Phys. Lett. 2001, 341, 425.

gold and silver films, is remarkably enhanced employing the so-called Kretschmann configuration.11 Osawa et al. pioneered the application of this technique for equilibrium and time-resolved studies at electrochemical interfaces.5 The surface-enhanced infrared adsorption effect is attributed to (1) a long-range electromagnetic mechanism (dominant) that involves the excitation of local surface plasmons within thin films of coinage metals and the polarizability perturbation of the metal by the adsorbate species12 and (2) a short-range chemical contribution similar to the charge-transfer mechanism in SERS.13,14 The magnitude of the enhanced IR signal, I, depends critically on island structure (size, shape, interparticle spacing, mass thickness) and chemical composition of the metal film.15 The surface selection rule of SEIRAS states that only molecular vibrations with dipole changes perpendicular to the surface can be observed, which implies that the exciting electric field is normal to the local surfaces of the metal islands at any point and that s-polarized light does not generate any detectable resonance.5,15 The surface selection rule is represented by the mathematical expression16

I ∼ Γ(dµ/dQ)2 |E2| cos2 θ ∼ Γ cos2 θ where Γ is the surface concentration and θ is the angle between the dipole moment derivative of the vibrational mode (dµ/dQ)2 and the electric filed that excites the molecule |E2|. Consequently, the observed spectral features can be directly ascribed to a specific interfacial orientation of the respective adsorbate. The application of ATRSEIRAS experiments at solid/liquid interfaces with well(11) Hartstein, A.; Kirtley, J. R.; Tsang, J. C. Phys. Rev. Lett. 1980, 45, 201. (12) Osawa, M.; Ataka, K.; Yoshi, K.; Nishikawa, Y. Appl. Spectrosc. 1993, 47, 1497. (13) Siemes, C.; Bruckbauer, A.; Goussev, A.; Otto, A.; Sinther, M.; Pucci, A. J. Raman Spectrosc. 2001, 32, 231. (14) Merklin, G. T.; Griffith, P. R. Langmuir 1997, 13, 6159. (15) Suetaka, W. Surface Infrared and Raman SpectroscopysMethods and Applications; Plenum Press: New York, 1995. (16) Cai, W. B.; Wan, L. J.; Noda, H.; Hibino, Y.; Ataka, K.; Osawa, M. Langmuir 1998, 14, 6992.

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defined metal films deposited on highly refractive, IRtransparent prisms has several advantages, in comparison to IRAS studies employing the so-called thin electrolyte film configuration (1-5 µm) between the optical window and the working electrode: (1) high and specific surface sensitivity with an enhancement up to ca. 100 times stronger as in conventional IRAS; (2) dominant first layer effect (Helmholtz region) with a short decay length of the enhanced field of 3-4 nm (Gouy region) toward the bulk electrolyte; (3) signals from the interface and the solution background are of comparable magnitude and can be separated by applying the potential difference tactics; (4) no severe limitations due to mass transport and potential perturbations.5,17 In this contribution we report an in situ ATR-SEIRAS study in combination with scanning tunneling microscopy (STM) on the phase behavior of 4,4′-bipyridine (4,4′-BP) and water coadsorbed from neutral aqueous solution onto quasi-(111)-oriented thin gold films. The bifunctional nonchelating ligand 4,4′-BP acts as a bridging spacer and coordination unit in 3D and/or 2D supramolecular lattices with novel electric and magnetic properties18,19,20 and operates as an electron shuttle in the electron transfer of cytochrome c on electrochemical interfaces.21 Michl et al. employed 4,4′-BP with its unique diode-like properties (lone-pair σ-electrons, π,π* orbitals) as a basic building block for “conductive” molecular rods to communicate between functional elements such as “molecular machines” and nanoelectrodes.19,22 The torsional angle between the two pyridyl rings ranges, depending on environment (soild, liquid, gas phase) and coordination state, between all planar, 18.5° and 37.2°.23-26 These values reflect the relative contributions of the π-electronic interactions of the pyridyl fragments (favorable to coplanar conformation) and the ortho-bonded interaction of these fragments (favorable to nonplanar structure). Impedance (capacitance), voltammetric, and electroreflectance experiments indicate a potential-induced reorientation of 4,4′-BP from flat to tilted or perpendicular on mercury,27 gold,28-31 silver,32,33 and platinum electrodes.34 Wilde et al. speculate, based on EC-quartz crystal microbalance experiments with Au(poly), that 4,4′-BP is coadsorbed with surface water and anions of the sup(17) Johnson, E.; Avoca, R. J. Phys. Chem. 1995, 99, 9325. (18) Steel, P. J. Coord. Chem. Rev. 1990, 106, 227. (19) Stoddart, J. F., Ed. Acc. Chem. Res. 2001, 34, 411 ff. (20) Chawla, S. K.; Hundal, M. S.; Kaur, J.; Obrai, S. Polyhedron 2001, 20, 2105. (21) Armstrong, F. A.; Hill, H. A. O.; Walton, N. J. Acc. Chem. Res. 1988, 21, 407. (22) Schwab, R. F.; Levin, M. D.; Michl, J. Chem. Rev. 1999, 99, 1863. (23) Boag, N. M.; Coward, K. M.; Jones, A. C.; Pemble, M. E.; Thompson, J. R. Acta Crystallogr., Sect. C 1999, 55, 672. (24) Spotswood, T.; Tanzer, C. I. Aust. J. Chem. 1967, 20, 1227. (25) Magutova, Y. S.; Maltseva, L. S.; Kamaer, F. G.; Leontev, B. V.; Mikhamedkhanova, S.; Otrosh, O. S.; Sadykev, A. S. Izv. Akad. Nauk USSR, Ser. Khim. 1973, 7, 1510. (26) Ould-Moussa, L.; Poizat, O.; Castilla-Ventura, M.; Buntinx, S.; Kassab, E. J. Phys. Chem. 1996, 100, 2072. (27) Akhmentov, N. K.; Kaganovich, R. I.; Mambetkaziev, E. A.; Damaskin, B. B. Electrochimija 1977, 13, 280. Akhmetov, N. K.; Kaganovich, R. I.; Damaskin, B. B.; Mambetkaziev, E. A. Electrochimija 1978, 14, 176. (28) Uosaki, K.; Hill, H. O. A. J. Electroanal. Chem. 1981, 122, 321. (29) Czerwinski, A.; Zamponi, S.; Sobkowski, J.; Marassi, R. Electrochim. Acta 1990, 35, 591. (30) Sagara, T.; Marakami, H.; Igarashi, S.; Sato, H.; Niki, K. Langmuir 1991, 7, 43. (31) Mayer, D.; Dretschkow, Th.; Ataka, K.; Wandlowski, Th. J. Electroanal. Chem., in press. (32) Cotton, M. T.; Varga, M. Chem. Phys. Lett. 1984, 106, 491. (33) Liu, T.; Cotton, T. M.; Birke, R. L.; Lombardi, J. R. Langmuir 1989, 5, 406. (34) Chaffins, S. A.; Gui, J. Y.; Khan, B. E.; Liu, C. H.; Salaita, G. N.; Stern, D. A.; Zapien, D. C.; Hubbard, A. Langmuir 1990, 6, 957.

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porting electrolyte.35,36 This idea was supported by a recent in situ ATR study of 4,4′-BP adsorbed under potential control from aqueous sulfuric acid solution onto a thin gold film.37 Surface coordination of 4,4′-BP in an end-on configuration was suggested from in situ SERS experiments on Au(poly),38 Ag(poly),32,33,39-41 and one FTIR study on several metal oxide surfaces.42 Similar conclusions were also drawn from electron energy loss spectroscopy (EELS) experiments, after transfer of a 4,4′-BP modified Pt(111) electrode into an ultrahigh vacuum (UHV) system.34 Structural details of the steady state and dynamic phase behavior of up to three ordered 4,4′-BP adlayers on Au(111) were described by Cunha et al.43 and Mayer et al.31 in two recent molecular-resolution in situ STM contributions. Umemura44 and Pinheiro45 et al. reported results of ex situ investigations on the selfassembly of 4,4′-BP on Au(111) from ethanol solution and its coadsorption behavior with phenanthroline and/or merccaptopyrimidine, respectively. On the basis of our previous STM work we will present in this contribution a comprehensive in situ ATR-SEIRAS study of the vibrational properties of 4,4′-BP and coadsorbed water species on quasi-single-crystalline Au(111) film electrodes from aqueous electrolytes. The infrared spectra will be compared with SER spectra and results of SFG experiments. 2. Experimental Section IR Spectroscopy. The SEIRAS experiments were carried out in a vertical spectroelectrochemical cell46 employing the Kretschmann attenuated total reflection configuration. The working electrode, a thin gold film on a highly resistive (6000 Ω cm) Si hemisphere (25 mm diameter), was prepared by electron beam evaporation at a base pressure of 2 × 10-7 mbar and a deposition rate of (0.005-0.007) nm s-1. The mass thickness, dm ) 20 nm, was measured with a quartz crystal microbalance. The corresponding optical thickness, do, was estimated by ellipsometry to 23.5 ( 0.5 nm. The gold film consists of preferentially (111) oriented islands with an average diameter ranging between 20 and 30 nm (Figure 1). The metal films were electrochemically annealed by cycling the potential in the double layer region with 50 mV s-1. A typical voltammetric response of the resulting film, as for instance obtained after 30 min in sulfuric acid solution, exhibits well-defined features for the lifting of the (p × x3) reconstruction (P1) and the disorder/order phase transition of an (x3 × x7) (hydrogen) sulfate overlayer (P2). These observations are indicative of a high area (35) Wilde, C. P.; Ding, T. J. Electroanal. Chem. 1992, 327, 279. (36) Zheng, X.; Hatton, R. Electrochim. Acta 2000, 45, 3629. (37) Futamata, M. J. Phys. Chem. B 2001, 105, 6933. Futamata, M. Chem. Phys. Lett. 2000, 332, 421. (38) Taniguchi, I.; Iseki, M.; Yamaguchi, H. J. Electroanal. Chem. 1985, 186, 29. (39) Wang, X.; Zhao, B.; Wang, Y.; Wu, Y.; Xu, W.; Fan, X. Mater. Sci. Eng., C 1999, 10, 3. (40) Miranda, M. Raman Spectrosc. 1996, 27, 435. Miranda, M.; Neto, N.; Sbrana, G. J. Mol. Struct. 1997, 410-411, 205. (41) Rubim, J. C.; Temperini, M. L. A.; Corio, P.; Sala, O.; Hubert, A. H.; Chacon-Villalha, M. E.; Aymonino, P. J. J. Phys. Chem. 1995, 99, 345. (42) Bagshaw, S. A.; Cooney, R. P. Appl. Spectrosc. 1996, 50, 310. (43) Cunha, F.; Tao, N. J.; Wang, X. W.; Jin, Q.; Duong, B.; DeAgnese, J.; Langmuir 1996, 12, 6410. (44) Umemura, K.; Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. J. Electroanal. Chem. 1997, 438, 207. (45) Pineirho, L. S.; Temperini, M. L. A. Appl. Surf. Sci. 2001, 171, 89. (46) For details see: Wandlowski, Th.; Diesing, D.; Ataka, K. Phys. Chem. Chem. Phys., in preparation.

4,4′-Bipyridine Adsorption on Gold

Figure 1. (A) Cyclic voltammograms of quasi-(111) Au (20 nm) film (solid lines) and stepped Au(15 15 13) (dotted lines) electrodes in contact with 0.05 M H2SO4, scan rate 10 mV s-1. The curve showing just the double layer region was recorded with 50 mV s-1 and is plotted in a magnified current scale as indicated. Panels B, C, and D show ex situ STM and AFM images of the Au (20 nm) film electrodes, as obtained immedeately after evaporation.

percentage of (111)-oriented terrace sites.47 The direct comparison of the oxidation/reduction fingerprint region with current vs potential curves measured with stepped single crystals, Au[n(111) × (110)], yields a rather comparable response of our film with Au[15(111) × (110)] (miscut angle of 3.8° with respect to an ideal (111)terminated surface), assuming a roughness factor of 2 (Figure 1). The infrared spectra were measured with a Bruker IFS 66V/s, choosing a spectral resolution of 4 cm-1. Unpolarized infrared radiation from a globar source was focused onto the electrode/electrolyte interface by passing through the silicon prism (cf. surface selection rules and ref 46). The incident angle was 70° referred to the surface normal. The radiation totally reflected at the interface was measured with a liquid nitrogen cooled MCT detector (MCT 316, Colmar Technologies). Typically, 200 interferograms were scanned and coadded into each singlebeam spectrum (200 kHz scanning rate of the interferometer). The spectra were plotted in absorbance units defined as A ) -log(I/Io), where I and Io represent the intensities of the reflected radiation at sample and reference potentials, respectively. The acquisition of spectra was performed either at constant potential or during a slow potential sweep with 5 mV s-1. The electrode potential was controlled and synchronized with a HEKA potentiostat PG 310. Scanning Tunneling Microscopy. The STM and atomic force microscopy (AFM) experiments were carried out with a Molecular Imaging Pico-SPM. The STM tips were electrochemically etched Pt/Ir tips (70/30, 0.25 mm diameter), coated with polyethylene. The in situ experiments were performed with a platinum wire and a gold (47) Dretschkow, Th.; Wandlowski, Th. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 749.

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Figure 2. Cyclic voltammogram for Au(111) (solid line) or Au (20 nm (111)) (dotted line)/0.05 M KClO4 in the presence of 3 mM 4,4′-BP, scan rate 10 mV s-1. In situ STM images represent various stages of the potential-dependent adsorption/phase formation processes: (A) electrochemically annealed Au(111)(1 × 1) surface at 0.20 V; (B) 4,4′-BP adlayer I at 0.17 V; (C, D) 4,4′-BP adlayer II at -0.23 and at -0.21 V; (E) organic adlayer at -0.32 V. The corresponding unit cells are indicated. The stability regions of the various adlayer phases are labeled I-IV.

oxide microelectrode as counter and reference electrodes, respectively. All STM images were recorded at room temperature in constant current mode with tunneling currents ranging between 3 and 200 pA. Single-Crystal Electrodes and Solutions. The smooth Au(111) or stepped Au(n n n - 2) ) Au[n(111)(110)] electrodes were either single-crystal cylinders (4 mm diameter, 4 mm height, EC) or disks (10 mm diameter and 2 mm height, STM). Before each measurement the massive gold single crystals were flame annealed in a butane flame at red heat and then cooled in high-purity argon. Contact with the electrolyte was established under potential control. All potentials in this paper are quoted with respect to the saturated calomel electrode (SCE). The solutions were prepared with Milli-Q water (18 MΩ cm, 3 ppb TOC), H2SO4 (suprapure, Merck), KClO4 (Fluka, puriss p.a., twice recrystallized from water). All electrolytes (EC, SEIRAS) were deareated with argon prior to and during each experiment. The measurements were carried out at 20 ( 0.5 °C. Details of the SERS and SFG will be reported elsewhere.48,49 3. Results and Discussion 3.1. Electrochemistry and Scanning Tunneling Microscopy. Figure 2 shows typical cyclic voltammo(48) Wandlowski, Th.; Lin, W. T.; Huong, Z.; Ren, F. R.; Tian, Z. Q. Chem. Phys. Lett., in preparation. (49) Wandlowski, Th.; Busson, B.; Vidal, F.; Tadjeddine, A., in preparation.

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grams of 3 mM 4,4′-BP adsorbed from aqueous 0.05 M KClO4 solution (pH ∼ 5.8) onto a perfect Au(111)-(1 × 1) single-crystal electrode (solid line, Figure 2) or onto a quasi (111) single crystalline 20 nm thick Au film (dotted line). The latter is supported on a silicon prism. 4,4′-BP molecules assume under these conditions in the bulk electrolyte their unprotonated form (pKa1 ) 3.5; pKa2 ) 4.950). Both sets of data indicate clearly the existence of four different potential regions labeled I-IV, which are separated by distinct charging current peaks P1/P1′, P2/ P2′, and P3/P3′. Remarkable is the large hysteresis between the positive and negative potential scans. The experiments with the massive single crystal exhibits nonFaradaic phase transitions, which are higher and narrower in comparison with those of the film electrodes. The center peak positions obtained with both types of electrodes are rather similar. These observations correlate with the smaller terrace size and higher defect density of the latter (cf. Figure 1). Similar trends were also reported for order/ disorder phase transitions in (hydrogen) sulfate adlayers47 and physisorbed uracil films on stepped gold electrodes.51 The stability of range I is delimited at positive potentials by the competing adsorption of OH-, and the onset of gold oxidation,52 and at negative potentials by the current peaks P1/P1′. Steady-state in situ STM images of adlayer I revealed a periodic array of bright features with characteristic unit cell dimensions of a ) 1.10 nm (≈4aAu), b ) 1.25 nm, and R ) 76.5° (Figure 2B). This result, the analysis of high-resolution contrast patterns and domain properties, as well as packing considerations, motivated the hypothetical interpretation of I as a densely packed monolayer of interdigitated, perpendicularly oriented 4,4′BP molecules (six molecules per unit cell, surface concentration ΓmI ) 6.9 × 10-10 molecules cm-2), which are stabilized by N-coordination with the positively charged electrode surface and lateral π stacking.31,43 The highdensity phase transforms upon negative potential excursion into a new striped pattern II (Figure 2C) according to a progressive hole nucleation process combined with surface diffusion-controlled growth.31 High-resolution STM images, such as Figure 2D, revealed additional structural details. Assigning tentatively each “elongated” blob to an N-coordinated 4,4′-BP molecule, one may imagine a pattern of four parallel stacking rows. We proposed recently a high-order commensurate phase with a ) 1.15 nm ()4aAu), b ) 2.5 nm, and R ) 74°, which contains nine molecules.31 The corresponding area per molecule and coverage were estimated as AII ) 0.29 nm2 and ΓmII ) 5.7 × 10-10 molecules cm-2. 4,4′-BP molecules assemble into a third ordered adlayer just after crossing the current peak P2 toward negative potentials. STM experiments at rather low tunneling currents yield a regular rhombohedral arrangement of dark (or bright) spots (Figure 2E), which could be resolved at higher tunneling voltages into individual 4,4′-BP molecules arranged in parallel stacking rows.31 The suggested unit cell with a ) 1.15 nm, b ) 1.3 nm, and R ) 71°contains three molecules with AIII ) 0.47 nm2 and ΓmIII ) 3.4 × 10-10 mol cm-2. The reduction of 4,4′-BP into its anion radical takes place at potentials more negative than P3/P3′.31,38 Capacitance experiments indicate incomplete desorption. Returning the negative-going potential scan before the (50) Encyclopedia for Chemistry, Basic Part II, 4th ed.; Chemical Society of Japan: Tokyo, 1993; Chapter 10. (51) Bare, S.; van Krieken, M.; Buess-Herman, C.; Hamelin, A. J. Electroanal. Chem. 1998, 445, 7. (52) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, N.; J. Electroanal. Chem. 1987, 228, 429.

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onset of 4,4′-BP reduction yields in -0.500 V < E < 0.250 V the quasi-steady-state voltammetric response as plotted in Figure 2. The three differently ordered organic adlayers can be repeatedly created and dissolved during multiple potential cycles. The electrochemical and STM experiments indicate that the transition between the high coverage 4,4′-BP adlayer I toward the phases II and III might be tentatively represented by the successive loss of parallel stacking rows, reducing the coverage from 6.8 × 10-10 mol cm-2 via 5.7 × 10-10 mol cm-2 to 3.4 × 10-10 mol cm-2. Unfortunately, the strong variation of the STM contrast pattern of the present system with the applied tunneling conditions, especially the tunneling current, iT, and the tunneling voltage, VT, as discussed in ref 31, do not provide an unambiguous understanding of the potential-dependent interfacial orientations of 4,4′-BP and their interaction with solvent (water) molecules and/or ions of the supporting electrolyte. These aspects shall be addressed in the following in situ SEIRAS study. 3.2. Vibrational Spectroscopy, SEIRAS. 3.2.1. Absolute Spectra at Constant Potential and Kinetics of Adlayer Formation. The SEIRAS experiment started with a freshly deposited 20 nm thick gold film as working electrode in contact with 0.1 M aqueous KClO4 solution. After the electrolyte was deoxygenated, the films were electrochemically annealed by cycling the electrode potential with 50 mV s-1 in the double layer region (-0.60 V < E < 0.60 V) until a steady-state response of the electrochemical and spectroscopic signals was obtained (cf. refs 53 and 54). Due to the incomplete desorption of 4,4′-BP and the onset of reduction to form the anion radical at negative potentials, absolute absorbance spectra were measured first in the stability range of phase I at constant potential (E ) 0.200 V). The adsorbate-free 0.1 M KClO4 solution served as in situ reference. After addition of 3 mM 4,4′-BP into the stirred electrolyte (argon bubbling) single-beam spectra, obtained from the average of 200 coadded interferograms, were acquired every minute. Positivegoing bands indicate accumulation of the species with respect to the reference, which is represented by the 4,4′BP-free electrolyte. Typical SEIRA spectra of such an experiment are plotted in Figure 3. A steady-state spectral response is obtained after approximately 20 min. The most prominent positive-going broad bands in the region 3200 cm-1 < ν < 3700 cm-1 and around 1656 ( 3 cm-1 (fwhm ) 87 cm-1) are assigned to OH stretching (νOH) and HOH bending (δHOH) modes of coadsorbed water species.53 The positive sign indicates that the intensity increases with respect to the reference spectrum, e.g., the adsorption of 4,4′-BP increases the amount of oriented, coadsorbed water molecules within the double layer region and/or facilitates the formation of strongly hydrogen-bonded interfacial water (Helmholtz, Gouy region).55 With the assumption of Gaussian band shapes, the OH stretching modes could be deconvoluted into three contributions, centered at 3605 ( 3 cm-1 (fwhm 99 ( 4 cm-1), 3484 ( 2 cm-1 (fwhm 214 ( 4 cm-1), and 3308 ( 2 cm-1 (fwhm 390 ( 8 cm-1), respectively. As a typical example, a set of fitting curves is plotted in Figure 3A. The narrow band around 3605 cm-1 indicates the presence of non-hydrogen bonded OH,55 while the broad υOH bands at 3484 and 3308 cm-1 are assigned to strongly hydrogen-bonded OH and a Fermi resonance between υOH and the binary overtone of δHOH.53 (53) Ataka, K.; Yotsuyangi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 10664. (54) Ataka, K.; Osawa, M. J. Electroanal. Chem. 1999, 460, 188. (55) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211.


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Figure 4. Molecular coordinate system (A) and normal modes of experimentally observed in-plane B3u and B2u vibrations, with the spectral assignment taken from refs 26, 59, and 60.

Figure 3. Series of “absolute” SEIRAS spectra of a highly ordered Au(111) surface in 0.05 M KClO4 as a function of time (indicated in minutes in (A) and (B)) after addition of 3 mM 4,4′-BP at E ) 0.20 V. The reference spectrum was measured at t ) 0 min in the absence of the organic molecule. A typical example of the deconvolution of the OH-stretching region into three Gaussian contributions is also indicated in panel (A). (C) Time dependence of the normalized intensities of δHOH, νOH, and ν1601 (4,4′-BP). The solid lines represent fits to eq 1.

The band center positions and ratios of the integrated intensities of these three contributions do not significantly change with the accumulation time of 4,4′-BP. The later was estimated as 5:33:62. Comparison with the corresponding spectral response of the adsorbate-free electrolyte53 indicates that 4,4′-BP modifies the network of hydrogen-bonded interfacial water species. Referring to the previously reported ability of heterocyclic molecules such as cytosine,54 pyridine,56 2,2′-bipyridine,57 or pyrazine58 to replace interfacial water, the present result is rather different and points to a strong hydrophilic character of 4,4′-BP adsorption on Au(111). The spectral features of water at the interface are also different from the spectrum of the bulk electrolyte, which implies that the background absorption of the bulk solution is completely subtracted. Expansion of the spectrum into the “fingerprint” region between 1000 and 2000 cm-1 revealed the bands (Figure 3B), characteristic of adsorbed 4,4′-BP, at 1065 (w), 1220 (w), 1484 (w), and 1601 cm-1 (s), which increase in intensity with increasing interfacial coverage of 4,4′-BP. No peak shifts were observed, suggesting that molecule-molecule and molecule-substrate interactions are constant during the adsorption process. These four bands were found at rather similar frequencies in the KBr spectrum of the molecule and are attributed to a ring in-plane deformation + ring stretching (ν18a), CH in-plane bending (ν9a), CH in-plane bending + ring stretching (ν19a), and a prominent (56) Cai, W. B.; Wan, L. J.; Noda, H.; Hibino, Y.; Ataka, K.; Osawa, M. Langmuir 1998, 14, 6992. (57) Noda, H.; Minoka, T.; Wan, L. J.; Osawa, M. J. Electroanal. Chem. 2000, 481, 62. (58) Cai, W. B.; Amanov, T.; Osawa, M. J. Electroanal. Chem. 2001, 500, 147.

ring stretching mode (ν8a).26,59,60 The corresponding Wilson notation is given in parantheses. Referring to the D2h symmetry of the “free” 4,4′-BP molecules all vibrations observed in the potential region of the adlayer I in 1000 cm-1 < ν < 4000 cm-1 represent in-plane B3u modes; no B2u modes were observed (Figure 4). The negative-going (loss) band around 1130 cm-1 is attributed to the replacement of interfacial ClO4- ions.53 The spectral range below 1000 cm-1 could not be analyzed due to the strong absorption of the silicon prism. Finally we note that similar observations were made at any other potentials within the stability range of region I. The time dependence of the normalized (with the corresponding steady-state response at t ) 30 min, E ) 0.200 V, as reference), integrated intensities of ν1601, the most intense vibrational mode of adsorbed 4,4′-BP, and the two water features, νOH and δHOH, is plotted in Figure 3C. ν1601, as a typical representative of the observed B3u modes of 4,4′-BP reaches saturation after 5 min, while the establishment of the steady-state response of both interfacial water modes, which exhibit rather identical time dependencies, requires approximately 20 min. These spectroscopically obtained correlations between relative intensity Ir and time t can be represented by a first-order kinetic equation

Ir ) 1 - exp(-kt)

(1)

with k as an apparent rate constant. Curve fitting of eq 1 to the experimental transients (solid line in Figure 3C) yields k(ν1601) ) 0.73 ( 0.03 min-1, and k(νOH, δHOH) ) 0.30 ( 0.02 min-1. These results indicate that the formation of the high coverage 4,4′-BP adlayer (with reference to the adsorbate free base electrolyte, cf. ref 31) is accompanied by major, and obviously even slower ordering of interfacial water molecules within the Helmholtz and Gouy regions, both of which are probed. The penetration depth of SEIRA in our configuration does not access further contributions from the bulk electrolyte.5 3.2.2. Potential Dependence of 4,4′-BP Adsorption on Au(20 nm-111). After a steady-state spectral response was reached in region I at 0.200 V, the potential was slowly scanned with 5 mV s-1 toward -0.500 V (limit of the nonFaraday adsorption range of 4,4′-BP31,43) and then back to 0.200 V. The “relative” in situ SEIRA spectra were recorded simultaneously, choosing the steady-state singlebeam spectrum of the high coverage 4,4′-BP phase I at (59) Kihara, H.; Gondo, Y. J. Raman Spectrosc. 1986, 17, 263. (60) Topacli, A.; Akyuz, S. Spectrochim. Acta, A 1995, 51, 633.

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Figure 5. Selected SEIRAS spectra of 3 mM 4,4′-BP adsorbed on an Au (20 nm (111)) electrode in 0.05 M KClO4 as a function of potential, measured simultaneously with a slow scan voltammogram (5 mV s-1, negative potential sweep): (A) overview spectra; (B) zoom-out of the low-frequency range; (C) contour plot. 200 single scans are averaged within each potential interval of 42 mV. The reference spectrum was measured at 0.20 V (phase I).

0.200 V as reference. The IR spectra were acquired sequentially at every 42 mV, by averaging of 200 individual interferograms. Parts A and B of Figure 5 illustrate typical sets of selected relative spectra as obtained during a sweep toward negative potentials. (We note that downward bands represent loss features with respect to the reference state.5) The contour plot in Figure 5C represents the spectral response of one complete voltammetric cycle. Clearly developed and monotonically growing loss features of interfacial water occur in the OH-stretching (3200 cm-1 < υ < 3700 cm-1) and the HOH-bending (1656 cm-1 shifts toward 1646 cm-1) regions as well as for the B3u modes of 4,4′-BP around 1601 (s) and 1484 cm-1 (w) with increasing negative potentials (Figure 5). The potential-

dependent integral loss intensities of υOH, υHOH, and υ1601 are plotted in Figure 6A. Careful inspection of the potential evolution of the prominent stretching mode reveals that upon approaching the voltammetrically determined transition potential P1, an initially splitted double band with centered positions at 1611 and 1601 cm-1 develops. After passing P1, only the loss feature around 1601 cm-1 increases further in regions II, III, and IV (Figure 5B). A similar splitting of other B3u loss bands could not be detected because of the rather poor signal-to-noise ratio for υ1484 and the other two B3u bands below 1250 cm-1. Nevertheless, this observation may indicate site-specific desorption and/or structural changes within the high coverage 4,4′-BP phase.


Figure 6. Potential dependence (A) of the integrated intensities of the ν1601 (B3u; O, b) and ν1402 (B2u; 0, 9) and (B) of the normalized interfacial water vibrations νOH (O, b), δHOH (0, 9). (C), (D) potential dependence of the band positions of ν44BP and δHOH. The open (filled) symbols represent data aquired during a positive (negative) potential scan.

Similar arguments were recently discussed for pyridine. Hoon-Koshla et al. pointed out that the in-plane ring deformation mode of N-bonded pyridine on Au(111) splits into a small band at ∼1599 cm-1 associated to defect sites, and a main band at 1593 cm-1 (lower wavenumbers!) representing pyridine on (111) terraces.61 Simultaneously and correlated to the loss features of interfacial water and of B3u 4,4′-BP modes, two new bands start to grow (positive!) at υ ∼ 1402 cm-1, and a weaker feature at 1527 cm-1 (Figure 5). Both could be assigned to in-plane B2u modes of adsorbed 4,4′-BP, specifically to the in-plane bending + ring stretching mode (υ19b, ∼1402 cm-1) and the ring stretching mode (υ8b, ∼1527 cm-1).26,59,60 The peak positions are independent of the electrode potential. These structural changes of adsorbed 4,4′-BP and coadsorbed water are reproducible during multiple cycling. (61) Hoon-Koshla, M.; Fawcett, W. R.; Cheng, A.; Lipkowski, J.; Pettinger, B. Electrochim. Acta 1999, 45, 611.

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Changing the direction of the slow potential scan (negative as illustrated in Figure 5, positive not shown) causes a significant hysteresis in the evolution of the respective vibrational modes. Figure 6 illustrates, as a summary of these observations, the potential dependence of the integrated intensities of the two most intense modes of adsorbed 4,4′-BP, υ1601 (B3u), υ1402 (B2u), and of interfacial water, υOH and δHOH. The position of the B3u mode does not change with potential in -0.600 V < E < -0.500 V, except of the already described satellite feature at 1611 cm-1 around P1. During the negative-going potential scan, the integrated intensity of the ν1601 loss band (B3u) decreases initially rather moderately (still in region I), followed by a steep decrease at E < -0.100 V (voltammetric peak P1, I f II), which slows somewhat down at E < -0.250 V (II f III) until it levels off at E < -0.400 V (P3, region IV). The subsequent positive-going scan is represented by an initial small slope, which increases in two slightly separate steps at E > -0.400 V (P3′, IV f III) and at E > -0.220 V (∼P2′, III f II) until the slope levels off at E > 0.050 V (∼P1′, II f I). The hysteresis of approximately 50 mV between the forward and backward potential scans and the changes in slope of the loss-intensity vs potential plots correlate nicely with peak position and hysteresis of the corresponding voltammetric response (Figure 2). Similar correlations were found for the ν1402 gain mode (B2u). The negative potential scan exhibits a steep increase of the integrated intensity in region II (E < -0.100 V, I f II), a moderate change in region III until the change levels off in region IV at E < -0.400 V (III f IV). The potential dependencies of the integrated intensities of νOH and δHOH are identical (normalized representation in Figure 6B) and reflect changes in the four stability regions of adsorbed 4,4′-BP, and around the various phase transitions, but do not strictly correlate with the exact positions predicted from the voltammetric experiments. Remarkable is the hysteresis of approximately 0.200 V between the responses of the negative and positive potential scans (e.g., four times that of the 4,4′-BP modes). The water relaxation appears to be more pronounced within the IR-probed interfacial region than the sole rearrangement of the 4,4′-BP molecules. Interestingly, the latter seems to dominate the charging current-potential or time response (voltammetry, chronoamperometry). Deconvolution of the complex νOH stretching mode, with the assumption of Gaussian line shapes, reveals three components centered at 3600 ( 7 (fwhm ∼ 110 ( 10 cm-1), 3484 ( 5 (fwhm ∼ 210 ( 7 cm-1), and 3315 ( 20 cm-1 (fwhm ∼ 330-370 cm-1), rather independent of the applied electrode potential, and with relative contributions of 5:35:60. These results are similar to those reported previously for the absolute spectra in region I. The center position of the δHOH bending mode shifts with potential from 1654 ( 1 cm-1 in region I (E > -0.100 V) to lower wavenumbers, 1646 ( 2 cm-1 in region IV (Figure 6D). This trend indicates a slight but significant decrease of interfacial hydrogen bonding. Compared with the perchlorate containing electrolyte in the absence of 4,4′-BP, the position of the δHOH mode occurs at negative potentials at higher wavenumbers (1646 cm-1 instead of 1612 cm-1 53), which clearly represents the promotion of hydrogen bonding in the presence of 4,4′BP. No change of the baseline or background reflectivity of the electrode was observed within the potential range of an ideal polarizable Au(20 nm)/0.1 M KClO4 + 3 mM 4,4′BP interface. The reflectance change in the IR region represents predominantly the optical properties of the metal substrate. In consequence, one may conclude that

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Wandlowski et al. Table 1. Characteristic Vibrational Modes of 4,4′-BPa A. IR (crystal), in Situ SEIRAS, and SFG Experiments mode (symmetry)

IR υ/cm-1 bulk crystal

SEIRAS υ/cm-1 Au (20 nm) film

υ12 (B3u) υ18b (B2u) υ18a (B3u) υ9a (B3u) Ω υ3-9b-15 (B2u) υ19b (B2u) υ19a (B3u) υ8b (B2u) υ8a (B3u)

992 1043 1074 1222

1065 1220

1323 1411 1490 1532 1598

1402* 1484 1527* 1601

SFG υ/cm-1 Au(111)

1230 1295 1345 (?)

B. Raman and in Situ SERS Experiments mode (symmetry)

Raman υ/cm-1 bulk crystal

SERS (0.20 V) υ/cm-1 Au(poly)/OxRed

Ag Ag (?) Ag Ag B1g B1g Ag Ag

1001 1076 1231 1302 1324 1428 1514 1606 (1615)

1017 1072 1222 1289 1504 1607

a The SEIRAS modes marked by “asterisks” were found in regions II, III, and IV. The SFG experiments have been limited to the range 1150 cm-1 < υ < 1450 cm-1. Characteristic individual spectra are plotted in Figure 7 and Figure 8.

Figure 7. Comparison of IR and Raman spectra of 4,4′-BP: (A) IR spectrum of crystalline 4,4′-BP; (B) SEIRAS spectrum of 3 mM 4,4′-BP in 0.05 M KClO4/Au (20 nm (111)) at 0.20 V; (C) Raman spectrum of crystalline 4,4′-BP (λ ) 632 nm); (D, E) SERS spectra of 3 mM 4,4′-BP in 0.05 M KClO4 on a roughened Au(poly) electrode48 at 0.20 and -0.60 V.

4,4′-BP interacts weakly with Au(20 nm) and does not change its optical properties upon adsorption. 4. Discussion 4.1. Molecular Orientation of 4,4′-BP in Region I. The free 4,4-BP molecule (crystalline, solution vapor phase) exhibits D2h symmetry with 54 fundamental vibrational modes. The IR (5B1u, 9B2u, 9B3u) and Raman modes (10Ag, 9B1g, 5B2g, 3B3g), cf. definition of xyz molecular axes as shown in Figure 4, are mutually exclusive. The four remaining Au modes are neither IR nor Raman active. The two pyridine rings contribute equally to the electronic and hence vibrational properties of the molecule. This situation changes if 4,4′-BP adsorbs on a metal surface31-33,37-39,41 and/or coordinates with metal ions.60 Monodentate substrate-adsorbate coordination via the nitrogen atom of one of the pyridine rings reduces the symmetry of the adsorbed 4,4′-BP from D2h to C2v, which predicts identical IR and Raman modes (cf. pyridine,56,61 2,2′-BP,57 or pyrazine58). The D2h and C2v fundamentals, referred to the coordinate system in Figure 4, are related as follows: B3u + Ag f A1; B2u + B1g f B2; B1u + B2g f B1; Au + B3g f A2. The SEIRAS spectra in region I (E > -0.100 V) exhibit four bands of adsorbed 4,4′-BP (ν1065 (ν18a ) (w), ν1220 (υ9a) (w), υ1484 (ν19a) (w), υ1601 (ν8a) (s); Figure 3B, Figure 7B), which could be assigned to in-plane B3u modes. They represent changes of the molecular dipole moment along the z axis (Figure 4). None of the B2u vibrations (dipole changes along the in-plane y axis), rather strong in the crystalline phase or in solution (υ1411 (ν19b), υ1532 (ν8b )

1532);60 Table 1, Figure 7A), was observed. In addition, the position of the ring stretching mode ν8b ) 1601 cm-1 (B3u), which is sensitive to the interaction of the pyridine nitrogen with electron acceptors,60,62 shifts, in comparison to the response of the pure solid sample, toward slightly higher wavenumbers (Table 1) indicating substrate adsorbate coordination.63 On the other hand, the small peak shift and the potential-independent background intensity suggest a rather weak interaction between 4,4′-BP and the positively charged gold surface. By application of the surface selection rule in SEIRAS that only normal components of the dipole change are IR active (cf. Introduction and refs 5 and 15), the preferential observation of B3u modes in region I is interpreted such that 4,4′BP is adsorbed via one nitrogen atom in an end-on configuration with the C2 rotation axis of the molecule being perpendicular to the surface or with the y axis parallel to the surface. No IR-forbidden modes, as for instance discussed in the case of pyrazine on rough gold surfaces, were found.58,64 Flat-lying and edge-on configurations are clearly discarded. The frequency region below 1000 cm-1, where out-of-plane modes occur, was not accessible in our IR experiments due to the cutoff of the silicon prism. Nevertheless, the tilted orientation can be excluded, based on in situ SERS experiments with polished48 or roughened (oxidation/reduction cycles)38 polycrystalline gold electrodes (Figure 7D,E). No out-ofplane Raman modes, B2g, were found. The SER spectrum in region I is dominated by the symmetrical vibrational modes Ag (Figure 7D,E), which correspond to a perpendicular end-on orientation of 4,4′-BP and strengthen the molecular packing model suggested from the STM experiments31 (Figure 9). These spectral features are similar to those observed on silver electrodes.32,33 Except for the interring stretching mode υ1289 (Ω) and for the ring mode ν1021 (62) Barker, D. J.; Buchleton, J. S.; Clark, G. R.; Cooney, R. P.; Richard, C. E. F. J. Mol. Struct. 1990, 239, 249. (63) Metiu, H. Prog. Surf. Sci. 1984, 17, 153. (64) Brolo, A.; Irish, D. E. J. Electroanal. Chem. 1996, 414, 188.


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flat but rather in slightly tilted orientation with reduction products remaining partially adsorbed on the surface.32,33,48 4.3. Comparison with Other N-Heterocyclic Molecules and Role of Coadsorbed Water. Macroscopic and molecular monolayer models65,66 represent the adsorption of solute (A) and solvent molecules at metal/ aqueous electrolyte interfaces by equations of the following type67

Abulk + nH2Oads T Aadsλ + nH2Obulk + λe-

Figure 8. Selected potential-dependent SFG spectra of 3 mM 4,4′-BP in 0.05 M KClO4 on Au(111).49

(ν12) all SERS-active Ag vibrations exhibit counterparts of B3u symmetry at rather similar wavenumbers in the SEIRA spectrum. This coincidence of SER and SEIRA spectra supports the symmetry reduction toward C2v implying that all vibrational modes become Ramanallowed (cf. pyrazine in ref 58). Despite the fact that the inter-ring stretching mode Ω ∼ 1289 cm-1 is not found in the SEIRA spectrum, we can conclude from the existence of a distinct resonance feature around 1295 cm-1, as reported in a recent visible-infrared in situ SFG experiment49 (Figure 8), that this mode is indeed IR active for adsorbed 4,4′-BP on Au(111) in region I. Other SFG-active resonances in the range 1150 cm-1 < υ < 1450 cm-1 were found around 1230 cm-1 (υ9a) and 1345 cm-1. The intensity decreases with more negative potential while the peak position remains rather constant. The nature of υ1345 is not yet clear.49 We recall that the detection of a SFG resonance requires as a necessary condition its simultaneous IR and Raman activity.3 4.2. Orientation of 4,4′-BP in Regions II, II, and IV. (1) The increasing loss intensity of all SEIRAS-active B3u modes (Figures 5 and 6), (2) the simultaneous occurrence of two new in-plane B2u vibrations, ν1405 (ν19b) (m), ν1527 (ν8b) (w), with the latter having also a weak counterpart in the SER spectrum,48 and (3) the absence of out-of-plane B2g SERS modes at ν < 1000 cm-1 demonstrate that the partial desorption of 4,4′-BP at E < P1 is accompanied by an in-plane tilt of the remaining surface-bound organic molecules in the yz plane. The N coordination between 4,4′-BP and the gold surface and the dominant lateral interactions between the adsorbed molecules (π stacking) seem to remain unchanged, as indicated by the potential dependence of the positions of the respective vibrational modes and of the baseline intensity. The exact tilt angle could not be determined. The potential dependence of the most prominent stretching band ν1601 (ν8a) suggests two possible “tilt steps”, which could be associated with the adlayers II and III. Both structures have been characterized in high-resolution in situ STM experiments31 (Figure 9B,C). The decrease of the Raman intensity of the symmetrical Ag modes with more negative electrode potentials correlates with this interpetation (Figure 6A). In addition, one may conclude from the existence of these modes at potentials even more negative than -0.600 V (Figure 7D, Figure 8) that the reduction of 4,4′-BP past P3 occurs not in a completely

(2)

λe- represents the transfer of λ electrons. Combined thermodynamic (chronocoulometry) and structure-sensitive studies with well-defined electrodes demonstrated that the adsorption of organic molecules is influenced by the properties of interfacial water (“hydrophobic effect”68,69), ions of the supporting electrolyte, structural properties of the solute (hydrogen bonding, π stacking, substrate-adsorbate coordination, ion pairing, ...), and the substrate surface (crystallographic orientation, material).70,71 The adsorption of monofunctional aliphatic compounds on gold surfaces has the character of weak physisorption, which is dominated by Au-H2O, A-H2O, and H2O-H2O interactions (hydrophobic expulsion68) modified by the interfacial electric field.72,66 On the other hand, adsorption between aromatic nitrogen-containing heterocyclic molecules and a gold electrode has the character of weak chemisorption (metal-ligand interaction), which involves predominantly either (1) interactions between the π electrons of the aromatic ring and the delocalized, polarizable free electrons in the metal or (2) a mixing of the N-lone pair orbitals with the electronic states of the metal.70 Depending on the applied electrode potential, the heterocyclic adsorbate may assume both (1) a π-bonded or flat-lying position with the aromatic ring parallel to the negatively charged surface and (2) N-bonded (vertical, tilted, and/or rotated) orientations. In the following we will discuss our results of 4,4′-BP in comparison to the interfacial properties of other Nheterocyclic molecules adsorbed on massive or thin, quasisingle crystalline gold electrodes. Flat orientations, in which the aromatic rings of the molecule are oriented parallel to the electrode surface, was reported for pyridine (Py),73,56 pyrazine (pz),74,75 and 2,2′-bipyridine (2,2′-BP)76,77,78 based on chronocoulometry, in situ STM, and ex situ XPS studies. In situ SEIRAS (65) Damaskin, B. B.; Petrii, O. A.; Batrakov, V. V. Adsorption organischer Verbindungen an Elektroden; Akademie-Verlag: Berlin, 1975. (66) Cf. reviews in: Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH: New York, 1992. (67) Koppitz, F. D.; Schultze, J. W.; Rolle, R. D. J. Electroanal. Chem. 1984, 170, 5. (68) Israelachvili, J. Intermolecular and Surface Forces; Academic Press: London, 1992. (69) Tabor, D. J. Colloid Interface Sci. 1977, 58, 2. (70) Lipkowski, J.; Stolberg, L.; Yang, D. F.; Pettinger, B.; Mirwald, S.; Henglein, F.; Kolb, D. M. Electrochim. Acta 1994, 39, 1045. (71) Interfacial Electrochemistry; Wieckowski, A., Ed.; M. Dekker, Inc.: New York, 1999. (72) Beltowska-Brzezinska, M.; Luszak, T.; Holze, R. Surf. Sci. 1998, 418, 281. (73) Stolberg, L.; Morin, S.; Lipkowski, J.; Irish, D. E. J. Electroanal. Chem. 1991, 307, 241. (74) Ianelli, A.; Merza, J.; Lipkowski, J. J. Electroanal. Chem. 1994, 376, 49. (75) Hamm, U.; Lazarescu, V.; Kolb, D. M. J. Chem. Soc., Faraday Trans. 1996, 92, 3785. (76) Yang, D. F.; Bizzotto, D.; Lipkowski, J.; Pettinger, B.; Mirwald, S. J. Phys. Chem. 1994, 98, 7083. (77) Cunha, F.; Tao, N. J. Phys. Rev. Lett. 1995, 75, 2376. (78) Dretschkow, Th.; Lampner, D.; Wandlowski, Th. J. Electroanal. Chem. 1998, 458, 121.

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Figure 9. Orientation (derived from SEIRAS experiments) and packing (STM31) models of 4,4′-BP on Au(111).

experiments,56,58,79 which only probe the Helmholtz and Gouy regions, revealed that the formation of the π-bonded adsorption state of all three molecules is accompanied by the replacement of water molecules from the electrode surface, in accord with the model expressed by eq 2. Detailed analysis of the bending mode of water, δHOH, as reported for pyridine,56 showed that the loss features consists of two components representing individual and hydrogen-bonded water molecules. Upon changing the electrode potential, all three heterocyclic molecules coordinate to the positively charged surface with one (Py, Pz) or, in a coplanar cis-configuration, with both nitrogen atoms (2,2′-BP). This type of adsorbate-substrate coordination reduces the molecular symmetry to C2v, and as a consequence all vibrational modes become Raman active.58,79 Comparable SERS, IRAS, SEIRAS, and SFG investigations showed that the high coverage phase of Py represents a slightly rotated and end-on configuration,56,61,80 while Pz and 2,2′-BP are both vertically oriented.58,79,81 These coordinated adsorption states are stabilized by attractive, lateral π-stacking interactions between adjacent aromatic rings giving rise to twodimensional ordered monolayers. Typical contrast pattern of in situ STM experiments revealed large domains of parallel molecular stacking rows separated by 0.9-1.5 nm with intermolecular distances of 0.38-0.45 nm and molecules tilted up to 30° from the normal of the chain (79) Noda, H.; Minoha, T.; Wan, L. J.; Osawa, M. J. Electroanal. Chem. 2000, 481, 62. (80) Hebert, P.; Le Rille, A.; Zheng, W. Q.; Tadjeddine, A. J. Electroanal. Chem. 1998, 447, 5. (81) Hoon-Khosla, M.; Fawcett, W. R.; Goddard, J. D.; Tian, W. Q.; Lipkowski, J. Langmuir 2000, 16, 2356.

axis.56,77,78,82 Py and 2,2′-BP form the same stacking pattern in neutral and in acidic solutions, e.g., under conditions where at least one nitrogen atom is protonated in the bulk phase.56,77-79 This result reflects the strength of the chemisorbed adlayer. Similar close-packed N-coordinated 2D stacking structures were also reported for 1,10′phenanthroline (Phen)83 and 2,2′-6,2′′-terpyridine (TP)84 on Au(111). All five molecules are coordinated to the metal surface through the nitrogen heteroatom and with the phenyl ring(s) directed toward the bulk electrolyte, which causes the hydrophobic hydration68,85 of the organic adlayer, accompanied with the displacement of surface water according to eq 2. This, mainly entropic phenomenon arises from the structural rearrangement of water molecules in the overlapping solvation zones as two (or more) organic adspecies approach each other.68 SEIRA spectra of Py, Pz, and 2,2′-BP indeed show pronounced water loss features in the OH bending (δHOH) and the OH stretching (νOH) regions with increasing adsorbate coverage.56,58,79 One also notices that the N-bonded adsorption causes (1) a shift of the prominent symmetric A1 molecular ring modes (in C2v symmetry) toward higher wavenumbers, compared to the corresponding vibrations of the free molecule, and (2) an increase in the baseline level of the spectra. The latter represents the background reflectivity of the electrode or, in other words, the optical properties of the metal.86,87 Both trends are more pronounced with (82) Andreasen, G.; Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. Langmuir 1997, 13, 6814. (83) Cunha, F.; Jin, Q.; Tao, N. J.; Li, C. Z. Surf. Sci. 1997, 389, 19. (84) Pinheiro, L. S.; Temperini, M. L. A. Surf. Sci. 2000, 464, 176. (85) Conway, B. E.; Mathieson, J. G.; Dhar, H. P. J. Phys. Chem. 1974, 78, 1226. (86) Ataka, K.; Osawa, M. J. Electroanal. Chem. 1999, 460, 188.


increasing strength of chemisorption of the adsorbate (Pz < Py < 2,2′-BP56,58,79). The adsorption of 4,4′-BP on Au(111) reveals some subtle differences, in comparison to Py, Pz, 2,2′-BP, Phen, and TP: The formation of the anion radical prevented an unambiguous conclusion on a possible flat orientation of 4,4′-BP at sufficiently negative potentials in the low coverage region.31,38 At E > -0.500, e.g., still at a negatively charged electrode, a first in-plane tilted N-coordinated adlayer is formed, which transforms into a densely packed, interdigitated stacking pattern of all-perpendicularly arranged 4,4′-BP molecules at a positively charged Au(111) surface in two discrete steps31 (Figure 9). The surface coordination is rather weak, as indicated by (1) the small upshift of the A1 ring frequencies in C2v geometry and (2) the potential independence of the baseline level (Table 1). The absolute SEIRA spectra reveal (cf. Figure 3) that the increase of the 4,4′-BP coverage is accompanied by an increase of oriented interfacial water, as compared with the adsorbate-free reference. The narrower bandwidth, ∼214 cm-1, of the OH stretching contribution for strong hydrogen bonding, υOH (3484 cm-1), and of the bending mode, δHOH at 1650 cm-1 with 87 cm-1 (fwhm), compared to bulk water (400 cm-1 at 3370 cm-1 and 100 cm-1 at 1648 cm-1) suggest an ordered water structure surrounding the 4,4′-BP monolayer. The second nitrogen, which faces the electrolyte, can directly participate in hydrogen bonding to the adjacent solvent molecules. This configuration is valid in all three potential regions of Ncoordinated 4,4′-BP. The hydrophilic adsorption, e.g., the co-deposition of densely packed 4,4′-BP and (at least) second-layer ordered water molecules is supported by (1) EQCM experiments, where Wilde et al. could not find any mass changes upon 4,4′-BP adsorption on gold film electrodes35 as well as (2) by the high sensitivity of the STM contrast pattern of 4,4′-BP in the N-coordinated phases to tunneling currents larger than 30 pA. (Steadystate imaging requires unusually large distances between sample and tip!)31 The classical picture, “substitution of solvent molecules by adsorbed organic species”, as represented by eq 2 is not valid. Remarkable structural changes of the electrolyte may occur within the entire interfacial region of density inhomogeneities (usually only two, at most three layers88) that parallel the formation of hydrophilic organic monolayers. The unique sensitivity (87) Futamata, M. Chem. Phys. Lett. 2001, 333, 337.

Langmuir, Vol. 18, No. 11, 2002 4341

and selectivity, and its exclusive penetration depth into the Helmholtz and Gouy regions,5 qualify SEIRAS as an ideal approach to explore these phenomena in detail. Futamata et al.37 reported, for instance, in an ATR-IR study that diprotonated 4,4′-BPH22+ adsorbs only in perchlorate containing electrolyte in direct contact with the gold surface, while more strongly adsorbed SO42- or Cl- ions expel 4,4′-BPH22+ into the second layer outside the contact-adsorbed anions. The adsorbate-induced coformation of ordered water in the electrochemical double layer region of Au(111)/electrolyte interfaces was also observed for Cu underpotential deposition (UPD) in H2SO487 (but not for Pb UPD neither in H2SO4 nor in HClO4 due to the complete discharge of the Pb2+!), the deposition of fumaric acid,89 and 4-mercaptopyridine.90 The common features of all these systems are hydrophilic groups (SO42-, -COO-, 〉N|) exposed to the electrolyte. We speculate (due to the current lack of experimental data) that many organic spacers with functional groups that act as “bait” to promote surface binding and operation of proteins like cytochrome c, such as carboxylates, sulfonates, phosphates or 4-pyridyl, and aniline-type amines, ArNH2,21 undergo hydrophilic adsorption on gold surfaces and induce a high order of interfacial water. If the hydrophilic groups are blocked, for instance, by surface coordination as in the case of cytosine86 or uracil91 on Au(111), classical hydrophobic adsorption and phase formation are observed. SEIRAS appears to be a valuable tool to develop these structural concepts further. Acknowledgment. The present work was supported by the Volkswagen Foundation (77/116), the EG through IC-014-00, and the Research Center Ju¨lich. K.A. is indebted to the Alexander von Humboldt Foundation for a Research Fellowship. The authors also acknowledge the support of Professor Ibach, the stimulating discussions with Professor Z. Q. Tian, Professor A. Tadjeddine, and Dr. D. Diesing, and technical assistance of J. Ikonomov. LA025585K (88) Spohr, E. Habilitation, University of Ulm, 1995. (89) Noda, H.; Ataka, K.; Wan, L. J.; Osawa, M. Surf. Sci. 1999, 427-428, 190. (90) Hara, Y.; Wan, L. J.; Noda, H.; Taniguchi, I.; Osawa, M. J. Electroanal. Chem., in press. (91) Pronkin, S.; Wandlowski, Th. In preparation.

In Situ Infrared Study of 4,4'-Bipyridine Adsorption on Thin Gold Films

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